This invention relates to a method of controlling the operation of a hydrocracker or catalytic dewaxer and, more particularly, to methods for controlling hydrocracking selectivity, stability of hydrocracker operation and reactor exotherms.
Hydrocracking is an established process in petroleum refining and in its commercial scale operation zeolite based catalysts are progressively gaining market share because of their higher activity and long term stability. Large pore size zeolites are conventional for this purpose, for example, zeolite X or the various forms of zeolite Y such as ultrastable zeolite Y (USY). Another zeolite which has properties consistent with those and which has been described as having a structure comprising the 12-rings characteristic of large pore size zeolite is zeolite beta and this zeolite has been proposed for use as a hydrocracking catalyst in EP 94827. Zeolite beta is notable for its paraffin-selective behavior. That is, in a feed containing both paraffins and aromatics, it converts the paraffins in preference to the aromatics. This phenomenon is utilized in the hydrocracking process disclosed in EP 94827 to effect dewaxing concurrently with the hydrocracking so that a lower bottoms product pour point is achieved concurrently with a reduction in the boiling range. Another application of the properties of zeolite beta is to dewax petroleum feedstocks by a process of paraffin isomerization, as opposed to the selective paraffin cracking produced by the intermediate pore size zeolites such as ZSM-5. This dewaxing is disclosed in U.S. Pat. No. 4,419,220 and an improvement on the basic zeolite beta dewaxing process is described in U.S. Pat. No. 4,518,485 in which the feedstock is first subjected to hydrotreating in order to remove heteroatom-containing impurities such as sulfur and nitrogen compounds prior to the isomerization reaction. During the hydrotreating process the organic sulfur and nitrogen containing compounds are converted to inorganic sulfur and nitrogen, as hydrogen sulfide and ammonia respectively. Cooling of the hydrotreater effluent and interstage separation between the hydrotreating and dewaxing steps enables the inorganic nitrogen and sulfur to be removed before they pass into the catalytic isomerization/dewaxing zone.
The prior art teaches the addition of nitrogen-containing compounds in dewaxing processes using amorphous or shape-selective zeolite catalysts for various purposes. U.S. Pat. No. 3,657,110 to Hengstebeck et al. discloses a process for hydrocracking nitrogen feedstocks over acidic catalysts such as silica-alumina wherein nitrogen-containing hydrocarbons are added at selected points along the hydrocracking zone so that the nitrogen content of the hydrocarbons in the hydrocracking zone increases in the direction of flow through the hydrocracking zone in order to control the rate of reaction along the hydrocracking zone. U.S. Pat. No. 4,251,676 to Wu discloses a selective cracking process for alkylaromatics which is carried out in the presence of ammonia or organic amines over an intermediate pore size zeolite catalyst. U.S. Pat. No. 4,158,676 to Smith discloses an aromatic isomerization process over shape-selective zeolites which uses added basic nitrogen compounds or their precursors to improve isomerization selectivity. British Patent 1,429,291, discloses a lube hydrocracking process in which various nitrogen-containing compounds may be added to the feed. U.S. Pat. No. 4,428,824 to Ward discloses preparing lubricating oils using a dewaxing or hydrodewaxing process in the presence of added ammonia over shape-selective zeolites such as ZSM-5. U.S. Pat. No. 4,743,354 to Ward teaches a method for preparing hydrodewaxed distillate over a shape-selective zeolite such as ZSM-5 wherein the effluent from a hydrotreater which may contain ammonia is passed to a dewaxer.
From this discussion it is clear that zeolite beta based catalysts may, under appropriate conditions, promote isomerization reactions in preference to cracking reactions or, under other conditions, cracking reactions over isomerization reactions. The balance between the various types of reactions which may occur is dependent upon a number of factors including the composition of the feed and the exact process conditions which may be used. In general, cracking reactions are favored by the use of higher temperatures and more acidic catalysts while isomerization reactions are favored by lower temperatures and the use of a hydrogenation/dehydrogenation component on the catalyst which is relatively active. Thus, isomerization tends to be favored by the use of a catalyst containing a noble metal such as platinum which is highly active for hydrogenation and dehydrogenation reactions, a zeolite which has a moderate acidity and the use of moderate temperatures.
Although these considerations indicate that it would be possible to carry out the desired types of reactions in a selective manner by varying the composition of the catalyst in accordance both with the feedstock available and the desired product, life in the refining industry is rather more difficult outside the laboratory. In a refinery, loading and unloading of catalysts from a reactor is an expensive and time consuming process and is to be avoided if possible. Similarly, feedstocks of the desired composition may not always be available and the product characteristics may change from time to time, depending on the demand for them. Thus, the realities of commercial refining require that a process should be capable of ready adaptation to different feedstocks and different product demands with the minimum of operating changes: in particular, catalyst changes should be avoided if possible. For these reasons, it would be desirable to find some means of modifying the activity and product selectivity of the zeolite beta and other zeolite catalysts so as to modify the yield structure of the catalyst and hence, of the process in which it is being used. If this could be done, it would be possible, for example, to process different feedstocks so as to effect a bulk conversion as well as a dewaxing or, alternatively, to carry out dewaxing by isomerization or to alter the selectivity to distillate or naphtha hydrocracking products. In the first case, waxy gas oils could be hydrocracked and dewaxed at the same time to produce low pour point distillate products such as heating oil, jet fuel and diesel fuel and in the second case, lubricant feedstocks could be selectively dewaxed by isomerization.
Another aspect of the use of zeolite based hydrocracking catalysts such as zeolite beta, zeolite X and zeolite Y which is of some importance in the refining industry is that they have a potential for temperature runaway under adiabatic reaction conditions, which may cause irreversible damage to the cracking catalyst and process equipment. Recent studies have shown that the high activation energy for zeolite-catalyzed hydrocracking process coupled with a relatively high hydrogen consumption, suggests that temperature runaway is highly plausible for a hydrocracker using a zeolite-based catalyst. The potential for harmful unexpected exotherms is particularly great when conditions are changed, e.g., feed composition is altered. In addition, excessive exotherms may arise under steady state conditions: the temperature at some point in the reactorxe2x80x94usually the back end, may be stable but too high for the desired degree of selectivity or cycle length.
Currently available schemes for controlling temperature runaway utilize quench hydrogen to lower the reactor l temperature in the high temperature stage. Hydrogen quench is effective for a normal operation with minor adjustment of reactor temperature but under potential temperature runaway situations hydrogen quench may be disastrous. This is partially due to the injection of additional hydrogen to the xe2x80x9chydrogen starvationxe2x80x9d temperature runaway zone. Another factor which has often been ignored is the wrong way behavior, resulting from the differences in the creeping velocity between mass and heat transfer waves. See xe2x80x9cChemical Reactor Design and Operation,xe2x80x9d Westerterp, Van Swaaij, and Beenackers, John Wiley and Sons, 1984. The injection of the quench hydrogen reduces the temperature and conversion near the inlet of the potentially dangerous stage. Under normal conditions, heat waves travel slower than mass waves. Consequently, the high temperature zone, which normally appears near the outlet of the stage for an adiabatic reactor, may be fueled with unconverted hydrocarbons entrained from the quenched zone. Eventually, the reactor will attain its lower temperature steady state. However, this dynamic response of the wrong way behavior using hydrogen quench may potentially induce irreversible deactivation for the cracking catalyst, e.g., sintering of the metal hydrogenation component. Damage to the process equipment, e.g., reactor and heat exchanger, resulting from the wrong way behavior, is possible. For this reason some alternative method of controlling hydrocracker operation including, in particular, temperature excursions, is desirable.
It has now been found that nitrogen compounds may be used to control catalyst activity, product selectivity and to control thermal behavior in an adiabatic reactor. In a particular application, it has been found that the selectivity of zeolite beta for isomerization may be improved by adding nitrogen containing compounds to the feedstock before or during the processing. This result is unexpected because it is known that nitrogen containing compounds are well known to be detrimental for the performance of zeolite catalysts. The selectivity for isomerization is reversible merely by discontinuing the cofeeding of the nitrogen containing compound so that if cracking performance should be desired again, it can be regained by reverting to operation without the nitrogen compound. Selectivity may be controlled in this way so as to maintain the desired product distribution: with lube boiling range materials, isomerization selectivity may be maintained at a desired high level to dewax without cracking out of the lube boiling range; in other applications, less isomerization selectivity may be required so as to isomerize and hydrocrack the feed to middle distillates but without overcracking; finally, isomerization selectivity may be minimized if the feed is to be hydrocracked all the way to naphtha. Appropriate adjustment of the amount of nitrogen compounds admitted to the reactor will enable the selectivity to be varied in this way.
According to the present invention, therefore, there is provided a method for controlling the operation of a hydrocracking process by the addition of a nitrogen compound or a precursor of such a compound to the hydrocracker feed or to the reactor. Suitable nitrogen compounds for this purpose include basic compounds such as amines, basic heterocyclic nitrogen compounds. In addition, nitrogen-containing petroleum refinery streams may also be used to provide the nitrogenous compounds, usually in the form of nitrogen-containing heterocyclic compounds, to control the operation of the hydrocracker.
In the application of the process to the control of isomerization and hydrocracking over zeolite beta, the feedstock is isomerized by contact with zeolite beta under isomerization conditions with a requisite amount of the nitrogen compound in the feed to control the activity and selectivity of the catalyst for isomerization of the waxy paraffins. If reversion to less selective isomerization performance is desired i.e. more hydrocracking with a greater degree of conversion to lower boiling product, it suffices merely to cease the cofeeding of the nitrogen containing compound and after a brief period of time, the former activity of the catalyst for non-isomerization reactions is regained.
The addition of nitrogen compounds at intervals along the length of the reactor may be useful for control of the temperature profile in the reactor as well as for maintaining stable operation. Provision for maintaining stable operation under conditions creating a potential for temperature runaway, e.g., feedstock change or perturbation of the feed preheat furnace, are significant safety and cost effective features of the invention. The injection of nitrogen-containing compounds to the inter-bed quench zones is capable of causing a rapid decrease in cracking rate, resulting in well-controlled reactor operation.
In another embodiment, the present invention relates to a method of controlling the stability of an isomerization dewaxing process in which a waxy hydrocarbon fraction is contacted under dewaxing conditions with a zeolitic dewaxing catalyst comprising zeolite beta in a dewaxing reactor having an inlet and an outlet. This embodiment comprises injecting ammonia vapor into the reactor to contact the catalyst in amounts sufficient to prevent temperature runaway or maintain operating temperatures in said dewaxing reactor below 900xc2x0 F. (482xc2x0 C.), i.e., temperatures at which the dewaxing catalyst sustains damage. This embodiment is particularly useful where the dewaxing catalyst inventory comprises noble metals whose hydrogenation/dehydrogenation function is related to sufficient dispersion throughout the catalyst. Failure of the feed pump or upset of the reactor feed furnace can result in temperature runaway which results in operating temperatures high enough to damage the dewaxing catalyst.
The injection of ammonia vapor to control reaction rate during incipient runaway conditions has been found to be extremely effective in isomerization dewaxing processes which exhibit high apparent activation energy. The presence of ammonia in the vapor phase allows a quick response of the catalyst surface throughout the isomerization reactor due to the short residence time of the vapor phase as opposed to liquid phase materials, e.g., liquid quench or bulky nitrogen compound injection.
Sample calculations for the response time (or residence time) between vapor and liquid phases obtained from a commercial catalytic isomerization dewaxing unit of 12,000 BPD are described as follows: